The present invention relates to an amorphous silicon color detector made from amorphous materials which by being applied with different bias voltages, will have different absorbencies for the three primary colors of red, green and blue, whereby the colors can be detected.
The present invention also relates to an amorphous silicon color image sensor which utilizes a plurality of the color detectors arranged in a linear array for sensing colors and images of documents, a scanning device for reading the output of the array of detectors, an A/D converter for converting the output to digital signals and a processor for processing the signals to output. The sensor can be used in a fax machine or a scanning machine.
The present invention further relates to a manufacturing method of the amorphous silicon color detector.
Presently known commercialized color detectors are mainly made of two types of materials, i.e., single crystal silicon and amorphous silicon. Single crystal silicon color detectors comprise two types of detectors, one comprises a photo-diode and a filter, and the other omits the filter, but is structured as a PNP type. The amorphous silicon detectors can provide the advantages of easy manufacturing process, reduced cost (because of low temperature process and because that the amorphous silicon can be deposited on large-sized substrates, an enlarged device can be made) and the amorphous material thereof can be grown on different substrates (such as glass, metal, ceramic and polymeric film). Therefore, the amorphous silicon is now more widely used in semiconductor devices. In particular, as the response spectrum of amorphous silicon is very close to that of the visible light and the amorphous silicon has a high sensitivity to light (i.e., the ratio of light/dark conductivity thereof is high) and a short response time of light, the amorphous silicon is deemed to be a good material for use as a light detecting material. Besides, the amorphous silicon has a very high absorption coefficient to light (.+-.10.sup.4 cm.sup.-1) in the visible light range. Please refer to FIG. 7, wherein curve A is a sensitivity curve of human eyes to light and curves B and C are the spectral response curves of amorphous silicon and single crystal silicon photodiodes to visible light, respectively. As can be seen in the figure, amorphous silicon has the maximal changes in relative sensitivity to light of wavelengths in the range between 400-700 nm. It means that the amorphous silicon has particularly high absorbance to light in a certain wavelength range, and hence is particularly advantageous for use as color sensing elements.
Amorphous silicon color detectors have been used in image sensors, solar cells, xerography machine and color sensors. Amorphous silicon color detectors are used to convert the light absorbed by the photodiodes or photoconductors into electrical signals for output.
At present, the most widely used amorphous silicon color detector has a filter plate on an amorphous silicon photodiode. As shown in FIGS. 8(a)-8(c), such color detectors mainly comprise three kinds of detectors, i.e., a visible light detector (see FIG. 8(a)), a single color detector (see FIG. 8(b)) and an integrated full-color detectors (see FIG. 8(c)). All these detectors comprise glass substrates 81, transparent electrodes 82, amorphous silicone 83, inner electrodes 84, lead frames 85 and resin housings 86. In order to achieve the object of a single color detection, the glass substrate 81 can be covered with a color filter plate 87 (as seen in FIG. 8(b)), or a filter plate 88 including the three colors of red, green and blue to reach a full-color detecting object (as shown in FIG. 8(c)). However, the above-mentioned detectors have a common problem, i.e., an additional filter plate is required, the structures thereof become more complicated and the cost thereof increases. To solve the above problems, an amorphous silicon photo-detector has been developed which needs no additional filter plate. Such detectors use a particular diode structure and according to absorbencies for different colors, are applied with different bias voltages thereto to distinguish the color.
The other type of amorphous silicon color detector omits the filter plate. The color detectors made by the method mainly comprise an a-Si:H (amorphous silicon) photo transistor and a-Si:H/a-SiC:H hetero junction and have a n.sup.+ -i-p.sup.+ -i-n.sup.+ structure, such as those proposed by Hsiung-kuang Tsai et al in a paper titled "An Amorphous SiC/Si Two/color Detector", IEEE Electron Devices Letters, Vol. EDL-8, No. 8; and by B. S. Wu et al in a paper titled "Amorphous Silicone Photo-transistor on a Glass Substrate" IEEE Transactions on Electron Devices, Vol. ED-32, No. 11, November 1985. The Chinese Patent No. 105721 filed by Si-Chen Lee, et al titled "Amorphous SiC/Si three-color detector" suggests a structure of the transistor-type as described above with an additional i layer added to the original i layers to form a structure of n.sup.+ -i-i-p.sup.+ -i-i-n.sup.+ which is as shown in FIG. 9. By being applied with bias voltages within the range of .+-.2 V, the detectors can distinguish three color lights of red, blue and green according to the absorbance of the junction to the lights. The spectral responses for the three lights are shown in FIG. 10. FIG. 10 shows the measured response under voltage bias of -2.0 V, -0.2 V and 2.0 V, respectively. When V.sub.CE =+1 V, the response to light having wavelength of 480 nm (blue light) is maximal, then blue light is detected; when V.sub.CE =-0.2 V, the response to light having wavelength of 500 to 600 nm is maximal, then green light is detected; and when V.sub.CE =-2 V, red light is detected.
However, the above transistor-type structure has the following disadvantages: (1) the structure is too complicated and the thickness of the p layer has to be controlled to about 100 .ANG., which is difficult to do during the manufacturing process; (2) the p layer has a high doping density which would most likely contaminate the adjacent i layer and would cause the i layer to become a p.sup.- layer and make the response to the spectrum poor; and (3) the structure disclosed in Chinese Patent No. 105721 is particularly complicated since it adds a second i layer to the first i layer, resulting in the structure of indium tin oxide (ITO)/n-i-i-p-i-i-n/metal. Different donors and energy gap densities are needed between the two i layers and during the manufacturing process. The conditions of the manufacturing process are controlled only by means of the flow ratio of the SiH.sub.4 /H.sub.2. Under such conditions, it is extremely difficult to grow i layers with different donor and obtain necessary energy gap density. The process parameters are dependent on the structure of the reaction chamber and the peripheral design of the reaction system. Therefore, it is difficult to control the process parameters to grow i layers with different donors and energy densities. Also, the specification of the Chinese Patent No. 105721 lacks the description concerning the manufacturing process of the i layers, no mention about what the relationship between the size of the energy gap and the conditions when the film is grown up. Obviously, it is not easy to control in manufacture.